Radiotelephony



March 22, 1927.

A. N GOLDSMITH ET AL HADIOTELEPHONY File y 26. 1922 auventoz ALFRED N.GOLDSMITH r I JUL 5 W \NBERGER fhemflu 1 3% in} Mm Patented Mar. 22,1927.

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ALFRED N. GOLDSMITH AND JULIUS VIEINBERGEB. OF NEVI YORK, N. Y.,ASSIGN'QRS T RADIO COBIEORATION OF AMERICA, A CGRPORATION' OF DELAWARE.

RADIOTEL'EPHONY.

Application filed May 26,

This invention relates to methods and apparatus for accomplishinglimited or restnicted communication between radio trans- 'n andreceiving stations. WVhile the may be generally useful under mstances,in radio signaling it is applicable to preventing unaureception in radiotelephony. I-n

broadcasting systems as used at present any v person having a radioreceiving set capable of receiving the wave length on which the music orspeech, is transmitted may derive full-benefit therefrom regardless. ofwhether such person or persons contribute toward the cost of operationof the broadcasting station, It is therefore desirable in some cases, tolimit the reception to authorized statiens, which type of service weterm narrowcast transmission.

It is also desirable to provide a method and apparatus inv whichthereceiver can be sold outright but which will permit the transmittingstation to enact a chargeat certain intervals from its subscribers.

The general objects of the invention are to fulfill the aboverequirements. The more particular objects of the invention will appearmore fully in the course of the following description.

The novel features which we believe to be characteristic of ourinvention are set forth with particularity in the appended claims. Ourinvention, itself, however, both as to its organization and method ofoperation will best be understood by reference to the accompanyingdrawing in which Fig. 1 is a diagrammatic representation of atransmitting and a cooperating receiving station utilizing theinvention.

Figs. 2 to 5 are dia rammatic representations illustrating certain kindsof electrical filter circuits applicable to narrowcast receivers.

In accordance with the invention we 45 add to the outgoing music orspeech powerful interfering tones which distort the transmission andwhich would spoil the quality of the reception in an ordinary reseiner,and theneliminate the interfering ea tonetones ear receivers. The toneor 5 nven ion 1922,. Serial No. 563,836;

tones may be added to the transmitted Wave in various suitable ways andeliminated from the receiver in any suitable manner. The addedinterfering frequencies may be practically any frequencies which willproduce an audible tone or tones in anordinary receiver, but preferablyfrequencies will be lected hic ar not l kely to. ccur requently in themusic or'speeclr which is to be transmitted. By: way of example the folling inte er g ton m y be sed hich may e sent; ntinuous v r a lar orirregular intervals 7 (a) Very low tones below the normal range offrequencies of; speech or music;

e. g. 30 cycles per second. I

High tones above the normal range of frequencies of speech or music; e.g. 2500 cycles and over.

(0) Tones in the normal range of musical or speech frequencies, butpreferably at frequencies between two of the] frequenciesused in thescale of international pitch such. as is employed by orchestras or any.other pitch employed, and prefer-ably also such frequencies; as arenotimporta-nt in speech for the case of speech.

((5) Two tones or two ranges of tones, separated by afew cycles persecond, so that they will produce marked beats.

(0) Two tones, or sets of tones at markedly discordant intervals.

(f) One or more tones, the frequencies of which are varied in anydesired fashion within any desired range or ranges, for example in thezones of frequencies midwaybetween the frequencies of tones in thescaleof international pitch.

(9) Any combination of the above.

To accomplish the reception of the transmitted waves any suitableacousticor 'electrical' filtering means may be used-t0 eliminatethe'added frequencies, but we prefer to use electrlcal filters in whichcase the requirements for receivnig waves transmitted in accordancewith; (a) to (9 will be as follows (a) A high pass fiflter for passinall free. quencies abo the injected lew tone and attai ema I lit!!! (b)A low pass filter for passing all frequencies below the injected hightone and cutting out the latter. 7

(0), (cl), (6) and One or more highly selective band filters for cuttingout the injected tone or tones, but not affecting appreciably the nearbynotes of the scale.

(g) An appropriate combination of the above. 7

Any appropriate receiving and transmitting circuits may be used inconjunction with the means for producing and eliminating an interferingtone or tones and it will be understood that the system of Fig. 1 ismerely representative of one of the many possible forms which may beused. Referring to this figure, 1 is a telephone circuit for impressingthe speech or music on the high frequency transmitting circuits. Theinterfering tone or tones may be produced and radiated in a variety ofways, either using the same or a different high frequency radiatingsystem. For simplicity the arrangement is illustrated in an arrangementin which the interfering source 2 is directly associated with thetelephone circuit, for example, by coupling thereto. The radio frequencycircuits shown comprise the am plifying and modulatingtube circuits 3and the oscillating feed back tube circuits 4 for producing themodulated high frequency waves in the antenna 5. These waves will bemodulated not only by the speech or music to be received but also by theinterfering tone or tones. The speech or music modulated in this mannerreceived by an or dinary receiving station will be of poor quality oreven only partly comprehensible and of little practical value forentertainment or other purposes.

The original quality may be completely restored for a subscriber bymeans of electrical or mechanical filters of suitable acousticproperties before actuating the telephone or other receiver. Thefiltering means may be arranged in any appropriate manner relative tothe receiver. The receiving circuits shown comprise a receiving antenna6, receiving circuit 7 and amplifying and detecting tube 8 havingitsoutput circuit coupled to the circuit containing the filtering means 9.The telephone or other receiver 10 is con nected after the filteringmeans. By appropriate selection of the frequency characteristics of thefiltering means substantially all of the injected undesired tones areeliminated while all of the desired music or speech passes through tothe receiver substantially unaffected.

Although our invention is not specific to the particular type of filteror electrical. circuits, in Figs. 2 to 5 a number of known circuits ofelectrical filters are shown by way of example. The circuit of Fig. 2,comprising for example, a number of meshes of inductance 11, in seriesand capacity 12 in parallel (usually termed a low pass filter) may byappropriate selection of the values of the inductance and capacity, hemade to act as a shunt for all frequencies above 5000 cycles and as arelatively low impedance for frequencies below this value in a wellknown manner. If an interfering tone above 5000 cycles is used this willnot be heard in the receiver, but all frequencies below this value willbe heard substantially perfectly, as this type of filter may beconstructed or adjusted to have a cut off frequency which isrelativelysharp. As practically all of the tones of speech or music are below thisfrequency the quality of the reception will not be impaired.

Fig. 3 shows a high pass filter comprising a number of meshes ofcapacity 13 in series and inductance 14: in parallel. If an interferingfrequency of 30 cycles is selected the filter will act as asubstantially infinite impedance thereto and only the frequencies of thespeech or music (which are mainly above this frequency) will passthrough to the receiver.

By means of the more complex filter of Fig. 4, which is of known typeswe may selectively eliminate a desired band of frequencies whichcorrespond to the injected. interfering tone or tones of c to g. Thearrangement of Fig. 4:, which has meshes of capacity 15 shunted acrossinductances 16, in series in the line, and inductance 17 in series withcapacity 18 across the line, may be adjusted to eliminate a narrow bandof frequencies. This band may correspond to a tone or tones between twonotes of the international pitch or other pitch used.

Similarly the band filter of Fig. 5, using meshes of inductance 19 inseries with capacity 20 in the line and inductance 21 in parallel withcapacity 22 across the line, may be adjusted to pass two wide bands offre quencies eliminating a band of frequencies between these.

By an appropriate selection of two or more of the filters two or morefrequencies differing widely from each other may be eliminated in casetwo or more widely different tones are used.

In the actual operation of the system each subscriber will be suppliedwith filtering means to be connected in his receiving cir cuits whichwill eliminate the frequency which the transmitter injects into thespeech or music which it transmits. This injected tone will be changedat the transmitter at regular times and the subscriber will then find itnecessary to purchase new filtering means which will be supplied only bythose in control of the transmitting station.

In order to make it diflicult to duplicate the filtering means, therebyavoiding payment of a subscription fee, we preferably pourcement aroundthe impedances which makes-it exceedingly difficultto discover andduplicate the combination as the destruction of the=cement block:without damaging the elements isa tedious and troublesome process.

Fromthe foregoing it will be seen that practically any receiving stationmay be readi y equipped for this type of narrow cast reception and theappartus for this purpose may be sold outright to those subscribing tothe desiredentertainment.

Having described our invention, what we claim is:

1. In a radio signaling system, a method of restricted communicationwhich consists in generating radio frequency oscillations, modulatingsaid oscillations in accordance with a signal current and a confusionalternating current, said confusion current having a minimum frequencyof 2500 cycles per second, and transmitting said modulated oscillations.

2. In radio signaling systems, a method of restricted communicationbetween transmitting and receiving stations, which consists ingenerating radio frequencies, modulating the radio frequencies inaccordance with said communication, modulating the transmission at aninterfering tone frequency differing from the frequencies generallyexisting in said communication, said interfering tone comprising beatnotes produced by two tones separated by a few cycles per second,filtering out the interfering tone at the receiving station andreceiving the desired communication.

3. In radio signaling systems, a method of restricted communicationbetween transmitting and receiving stations, which cossists ingenerating radio frequencies, modulating the radio frequencies inaccordance with said communication, modulating the transmission at asinterfering tone frequency intermediate to two of the note frequenciesof a standard musical pitch, filtering out the interfering tone at thereceiving station and receiving the desired communication.

4. In a. radio signaling system, the combination of a transmitter, meansfor generating an interfering tone of a frequency differing from that ofa tone of standard musical pitch, and a receiver having filtering meansfor eliminating said interfering tone.

5. In a radio signaling system, the combination of a transmitter, meansfor modulating the radiation from said transmitter in accordance with adesired communication and at a frequency correspondin to an interferingtone of a frequency didering from that of a tone of standard musicalpitch, and a receiver having filtering means for eliminating saidinterfering tone.

6. Ina radio=signa=lin s-stem the combinatioir'of a radio telephonetransmitter,

means for radiating an interfering tone of a frequency differing fromthat of a tone of standard musical pitch, and. a receiver havingfiltering means for eliminating said interfering tone.

t. bination of a radio telephone transmitter, means for radiating aninterfering tone of a frequency differing from that of a tone ofstandard musical pitch, and a receiving station having an electricalfilter adapted to eliminate said interfering tone.

8. A radio telephone transmitting station for restricted communicationhaving means for modulating radiations in accordance with a desiredcommunication, and means for introducing in the transmission anauxiliary effect adapted normally to produce andible tones interferingwith the comprehension of the reception of the desired communication,said audible tones consisting of frequencies varied in a rangeintermediate of the frequencies of tones in the scale of internationalpitch.

9. A radio telephone transmitting station for restricted communicationhaving a single means for modulating the radiation in accordance with adesired communication, and means for introducing in the transmission anauxiliary effect adapted to normally produce audible tones interferingwith the comprehension of the reception of the desired communication,said audible tones consisting of frequencies Within a range offrequencies intermediate of that of tones in the scale of standardpitch.

10. A radio telephone transmitting station for restricted communicationcomprising a pair of radio telephone transmitters, operating onsubstantially the Same wave lengths, one being modulated by normalcommunication and the other by interfering tone of a frequency differingfrom that of a tone of a standard musical pitch.

11. In a radio signaling system means for generating a radio frequency,means for modulating said radio frequency in accordance with an audiofrequency, means for modulating said radio frequency in accordance witha confusion frequency having a frequency component within the range ofthe audio frequency and difiering from the frequency of a tone of astandard musical pitch, and means for transmitting said last modulatedradio frequency.

12. The method of receiving high frequency oscillations which have beenmodulated in accordance with an audio frequency and also in accordancewith a confusion current having a frequency component within the rangeof audible frequencies and differing from the frequency of a tone of astand- In a radio signaling system, the com and musical pitch,comprising demodulating said oscillations, and filtering said confusioncurrents from said audio frequency.

13. In radio signaling systems, a method of receiving restrictedcommunications comprising the steps of delnodulating the radio frequencyto produce an audio frequency band having a range of frequenciesincluding the restricting frequency, said restricting frequencydiffering from the frequency of a tone of a standard musical pitch,filtering out said restricting frequency, and rendering thecommunication intelligible.

ALFRED NQGOLDSMITH. JULIUS WEINBERGERL

